Recently installed 1 to many carbs on my 235 I started it up and I attempted to adjust the first carb when I turned the air adjustment screw it didn't seem to make much of a difference turned in or out . I did try to use a uni sync tool on the 2 carb I was able to get the bubble in the middle of the uni sync tool. on the first carb I couldn’t get it to move from the top of the tool. Would there be any benefit to split the vacuum advance line to hook up to both carbs? Any pointers that could be helpful. Carbs are Rochester’s with a manual choke Thanks
Which carbs are you running? Unless you have downsized the carburetor(s) considerably from a single one, you'll never be able to get a good idle mixture adjustment. For instance, two carbs for a 134 cubic inch 4-cylinder CJ-2 Jeep will only be a little over-carbureted (268 cubic inches) for a stovebolt engine, not MASSIVELY too much, like two 235 carbs (470 CI). Even running a pair of 216 carbs would help the situation a little. A couple of Motorcraft carbs for a 170 cubic inch Ford Falcon engine might work OK.
A Uni-Syn tool measures airflow, not mixture. It needs to be used with all the throttle linkage disconnected, and the throttle bell cranks held against their stops with small springs or rubber bands. Once the airflow is set correctly on both carbs and the throttle linkage has been reconnected without changing the idle speed, then you can tinker with the mixture screws. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Which carbs are you running? Unless you have downsized the carburetor(s) considerably from a single one, you'll never be able to get a good idle mixture adjustment. For instance, two carbs for a 134 cubic inch 4-cylinder CJ-2 Jeep will only be a little over-carbureted (268 cubic inches) for a stovebolt engine, not MASSIVELY too much, like two 235 carbs (470 CI). Even running a pair of 216 carbs would help the situation a little. A couple of Motorcraft carbs for a 170 cubic inch Ford Falcon engine might work OK.
A Uni-Syn tool measures airflow, not mixture. It needs to be used with all the throttle linkage disconnected, and the throttle bell cranks held against their stops with small springs or rubber bands. Once the airflow is set correctly on both carbs and the throttle linkage has been reconnected without changing the idle speed, then you can tinker with the mixture screws. Jerry
Unisync gauges measure air volume. With two oversized carburetors, the volume each has moving through it will be so low, the unisync tool won't be able to pick it up. It will almost be shut completely just to raise the ball a little bit. When its shut that much, you are now changing the mixture by choking the air flow. I ran twin Monojets on a 250 for a couple weeks. One was more then enough to power the engine, two worked really well at full throttle only. With low air volumes, the mixture was extremely rich, thus draining the fuel tank way faster then I liked. It was using so much fuel I could see the fuel gauge moving! No amount of leaning out or tuning will fix that situation. I now run dual Carter W-1's off 216 ci engines on a 250. They are just right and work really well for how I have them set up. I still can't use the unisync, my engine idles at 650 rpm and doesn't move enough air to make the tool work as it should. If both carbs are identical, set the throttle blades to 100% closed. Back the idle screw way off so you know for fact the throttles are closed, then bring each stop back in till it just touches the throttle lever. When you have both just touching, turn each screw one turn to open the throttles. You will have to start the engine at this point to set idle rpm. When you adjust one, adjust the other the same amount, it takes time and patience to get it right.
I totally can’t believe this I went to attempt to adjust my carbs and the fuel pump went out and yes I did check the fuel tank for fuel.now have to hunt for a fuel pump the is the 2nd glass bowl type of pump next one will just be a plain pump. Unbelievable don’t even know what to think.
With a little bit of creative tinkering, you can turn one of those failed glass bowl pumps into a dummy, and use an electric pump with an oil pressure operated failsafe system to actually supply the gas. Hide the electric pump back under the cab where it will be hard to see. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
The first one lasted for 20 years and actually didn't stop working. I replaced it in what some might call a preemptive move and kept it in case I needed it. Here's the nice part. This pump provides between 2.5 and 4 psi. That is honestly all you need for the 235. More than that and you're just going to stress your needle/seat and probably cause flooding. And by the way, I'd suggest finding a nice Carter YF carb and returning to a 1 carb approach. The Carter YF3211s is nice and runs a bit leaner than most of the YF models. It is an auto choke, but you could convert to a manual choke if you thought it was necessary.
Oh, and I didn't just put a block-out plate where the mechanical pump was...seemed sort of a waste of an opportunity to me. I ran lines from inside to outside of the engine and then to a remote oil filter. You can see an image attached. If you need more info on how I did that, please let me know. Good luck.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Rochester carbs are a bit of a challenge to get set up on a multi carb intake. Not impossible, but its a good bit of work. The very best outline of how to do it was written quite a few years ago by Pat Smith for Inliners. Pat was Dyson Racing's Crew Chief for many years, taught automotive mechanics, lots of stuff. Is one of the best mechanics and problem solvers I had the pleasurer of being introduced to. Here is his article:
As for the unisyn, you are not using it to adjust air fuel at the air fuel needles. You are using it to get the throttle blades in the same relative position so that each carb is taking in the same air at idle. Essentially, you want to make sure the throttle blades are not exposing the off idle circuit at idle or that one of the carbs has a throttle blade that is exposing the off idle circuit at idle. Otherwise you end up with the carbs going in and out of idle to off idle to idle and back to off idle as one carb leads the other in and out of the idle off idle circuits. So start by setting air fuel needle screws about 1 and a half turns out from bottom. Disconnect one of the carbs from the throttle linkage. Get the motor running. Install the unisyn on the carb with the linkage still attached. Get the ring centered in the vacuum tube. Now move it over to the other carb. Adjust the idle speed screw in or out until the ring is in the middle of the tube so that both carbs are now taking in the same amount of air at idle. If the idle is too high, turn the idle speed screws out (both carbs, a little at a time) or vice versa until you get to the lowest idle speed the motor will maintain. Double check each carb with the unisyn. Make adjustments if necessary. Now you can tinker with the idle mixture screws to get your best lean idle. Not an easy task with these Rochesters. Adjusting the idle mixture screws may very well raise or lower the idle speed. Readjust the idle speed screws as necessary to get the idle back to your idle speed. When the air fuel mixture is adjusted, recheck the carb with the unisyn.
The other alternative is to get a couple of adapters from Tom Langdon, and purchase a pair of 1974 ford pinto carbs (Holley/Weber 5200). Nice progressive carbs, run great out of the box and dial in way better then the Rochesters.
I got the new fuel pump in started up the truck idle was fine but upon acceleration Fist carburetor would pop with hesitation was messing with idle screw seemed to get a nice even balance but still hesitation and popping. Will try one more time than I think a Fenton duel carburetor intake will be for sale Thoughts or ideas needed. Thanks
Martin '62 Chevy C-10 Stepside Shortbed (Restomod in progress) '47 Chevy 3100 5 Window (long term project) ‘65 Chevy Biscayne (Emily) ‘39 Dodge Business Coupe (Clarence) “I fought the law and the law won" now I are a retired one! Support those brave men/women who stand the "Thin Blue Line"! Hug a cop! USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)
Too many carbs, and/or carbs that are WAY too big for the application. You've got low manifold vacuum, venturi velocity too low, and a lean backfire on acceleration. Congratulations on reinventing the wheel- - - -your results have been shared by untold thousands of stovebolt owners who don't accept that the laws of physics are not "suggestions"! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
As Jerry mentioned above, either too many carbs (or one too few ) that are oversize for the engine.
What you have definitely is not the ideal carburetion system; but several enthusiasts have made it work (sort of).
So, before throwing rocks at your Fenton:
(1) Did the engine run well prior to installing the dual carbs? (2) Have you done a compression test on the engine? (3) Have you verified that the advance mechanism in the distributor is functional, AND correctly adjusted? (4) Do you still have points and condenser, or has someone talked you into a DOWNGRADE of an electronic whizbang for the ignition? If points, have they been set prior to timing? Is the condenser new? (5) What is the value of your idle vacuum? Have you checked for a vacuum leak?
As to the failure of the Uni-Syn??? Over 60 years of playing with multiple carbs, I have adjusted hundreds using a Uni-Syn, and to date have never found an at-least semi-healthy engine to fail to allow readings; however, I must admit I have yet to even think about the consideration of trying one on dual Rochester B's.
And even though I believe you can get better carburetion by hiring a 15-year-old to stand on the running board and pour gas into the engine from a leaky boot than one B (let alone two), I personally believe you are looking at the wrong culprit!
Suggest you start with the basics: (1) Compression test, (2) Ignition test, and then think about the carb(s).
Jon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
As Jerry mentioned above, either too many carbs (or one too few ) that are oversize for the engine.
What you have definitely is not the ideal carburetion system; but several enthusiasts have made it work (sort of).
So, before throwing rocks at your Fenton:
(1) Did the engine run well prior to installing the dual carbs? (2) Have you done a compression test on the engine? (3) Have you verified that the advance mechanism in the distributor is functional, AND correctly adjusted? (4) Do you still have points and condenser, or has someone talked you into a DOWNGRADE of an electronic whizbang for the ignition? If points, have they been set prior to timing? Is the condenser new? (5) What is the value of your idle vacuum? Have you checked for a vacuum leak?
As to the failure of the Uni-Syn??? Over 60 years of playing with multiple carbs, I have adjusted hundreds using a Uni-Syn, and to date have never found an at-least semi-healthy engine to fail to allow readings; however, I must admit I have yet to even think about the consideration of trying one on dual Rochester B's.
And even though I believe you can get better carburetion by hiring a 15-year-old to stand on the running board and pour gas into the engine from a leaky boot than one B (let alone two), I personally believe you are looking at the wrong culprit!
Suggest you start with the basics: (1) Compression test, (2) Ignition test, and then think about the carb(s).
Jon.
1)engine ran great before duel carbs 2) vacuum advance was replaced 3) still have points & condenser 4) never done compression test 5) 216 carburetors with 235 base So going to save myself a lot of time and headache and install the stock intake manifold.
Swapping throttle bodies can be a tricky business. I prefer altering the stud spacing of the manifold by plugging and re-drilling the stud holes for the original 216 Carter carburetors. Even those carbs are massively too big when running them on a 235 engine.
"going to save myself a lot of time and headache and install the stock intake manifold"
Smart move- - - - -some people are too stubborn to admit they made a mistake. Good luck! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Swapping throttle bodies can be a tricky business. I prefer altering the stud spacing of the manifold by plugging and re-drilling the stud holes for the original 216 Carter carburetors. Even those carbs are massively too big when running them on a 235 engine.
"going to save myself a lot of time and headache and install the stock intake manifold"
Smart move- - - - -some people are too stubborn to admit they made a mistake. Good luck! Jerry
Yup I’ll own it seems to be what I do but knowing how well the engine ran and preformed I’ll put the stock intake back on or at least the correct one before it was a 216 intake with split manifold on the 235 still ran great just flat lined on power on the freeway around 55mph so stay tuned for next adventure
Quote "(5) 216 carburetors with 235 base" End quote
Bingo!
This makes poor venturi air velocity even worse!
The single carb now makes more sense (especially if it is upgraded to a Carter).
Jon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
Last question on this than I will put this dead horse to rest. With a 216 Rochester carburetor with a 235 throttle body on it should I put the 216 throttle body back on and drill out the difference to fit the stock 235 intake manifold? Or will it work with the 235 throttle body 235 manifold?
If you are going to use one carb on a 235, then use a 235 carburetor. A single 216 Rochester B on a 235 will significantly limit the capability of the engine.
If you are going to use two carbs on a 235, then:
(1) Fill the threaded holes in the intake, redrill and retap for the smaller flange.
OR
(2) Slot the throttle body
(Opinion) the only reason for putting two carbs on the 235 is "eye candy". Slotting the throttle body is more like "eye xxxx"
Jon
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
Dragsix is spot on regarding the dual Weber type carbs. I installed a pair of Langdons progressive two barrels. They work great! Incidentally, I am currently doing a S10 conversion and will soon no longer need the fenton dual intake, carbs, carb adapters and linkage. Let me know if you are interested in these.
1954 3100 with Hydra-Matic Currently undergoing S10 conversion Parkinson’s...it’s not just for the elderly Caution, slow man at work
There's no secret. A single tap gives you one vacuum signal. Two taps (at the base of each carb) gives you two vacuum signals, each slightly different. Then you need to decide which one the distributor needs to use.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
It really doesn't matter if you use one port, two ports, or no ports at all- - - - -the vacuum advance will seldom, if ever work. There will be such a low venturi velocity and such low manifold vacuum that the only time you'll get any movement of the advance will be when you're coasting downhill with enough throttle opening to expose the port in the throttle body that connects with the advance tubing. As soon as the vehicle starts to accelerate on a straightaway or even a slight uphill, the intake vacuum will be so low that the advance won't operate. Just plug the ports, forget about the vacuum line, and try to adjust the total advance for the best performance without using the vacuum advance. Problem solved, more or less! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
on the Rochester's, the vacuum advance operates from a ported source so generally there is no vacuum from the port at idle. That is why you don't necessarily have to disconnect the vacuum advance on a 235 to set the timing. This is a slightly different set up then for most other motors. Once the throttle blade begins to open, the port is exposed and vacuum is now created at the port and the advance begins to operate. The question is whether the motor is making enough vacuum to operate the advance which is something you have to test because the advance mechanism is set up to advance the timing at certain inches of vacuum depending on the movement and relative position of the diaphragm.
I never had any issues getting sufficient vacuum to operate the advance when running dual Rochester Bs or the Carter YFs I ran for quite a few years. I did have trouble getting enough vacuum from the dual Holley/Webers I use to run and the dual Weber 38s I currently run so I switched to full manifold vacuum (which presents its own set of issues but that is what hot rodding a motor is all about).
So I would run a single vacuum line and then check the operation. A typical 235 advance for say a 55 distributor, the diaphragm begins to move at between 4-6 inches of vacuum. The total advance is 6.5 to 8.5 distributor degrees running from 7 through 10 inches of vacuum. So that is what you will want to check to see if you have sufficient vacuum to support full vacuum advance.
Ok today I took apart a carburetor that was on the engine when I got it I took out the jet cleaned it up it has 58 stamped on it. this Rochester has electric choke. What size jets should I be looking for as far as stamped number on the 2 too many carbs right now 1 barrel Rochester manual choke ? Another website a guy said he thinks the guy that rebuilt his Rochesters might have put 48s in his carburetors I’m hoping one carburetor just mine have a different size jet causing the popping in the first carburetor.
If you're looking at vacuum at the advance, do it during a road test. Put a T fitting in the distributor line and run a long piece of tubing into the cab where you can watch the vacuum gauge during actual driving conditions. I think you're going to be amazed at how seldom there's enough vacuum present at the advance unit to actually make it move. That's particularly true if you've got two carbs, since even a tiny bit of throttle opening kills almost all the vacuum.
For jet sizing, ignore the stamped numbers- - - - -They don't actually represent jet diameter. Invest in a set of number drills, 0 to 60, and use the shank of a drill bit, not the cutting end, to check the jet diameter. A bigger hole equals more gas- - - -richer mixture. That's also dependent on throttle opening, because as soon as you tip in a little throttle, the power valve opens up and bypasses the main jet for a "full power" mixture. Once again- - - -more carbs- - - -lower vacuum- - - -more time spent with the jets bypassed. Ideally, multi-carb systems really should have weaker power valve springs, but I don't know of anybody who makes a selection of different-tension springs for the Rochester B. GM definitely didn't!
Tuning a Rochester B is sort of like herding cats- - - -pretty frustrating! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Start with equal jets in both. Stock jetting will be fairly close, close enough to run anyway. Have you had a vacuum gauge on the engine while it's running?
Start with equal jets in both. Stock jetting will be fairly close, close enough to run anyway. Have you had a vacuum gauge on the engine while it's running?
iatroca 52 Why don't you begin by synchronizing your throttle plates with a feeler gauge. Just reach in thru the air horn get both throttle plates the same gap. Then see how it idles,work from there.
What size jets should I be looking for as far as stamped number on the 2 too many carbs right now 1 barrel Rochester manual choke ? Another website a guy said he thinks the guy that rebuilt his Rochesters might have put 48s in his carburetors I’m hoping one carburetor just mine have a different size jet causing the popping in the first carburetor.
If you're talking about 1 Rochester on one relatively stock 235, it could have been anything from 41 to 84, but 56, 57, 58, 59 are in my observation the most common...OR some yoyo might have gotten all smart and drilled your jet out to who knows what pretending to be an engineer. A couple of years ago I sold all the Rochester B parts I had collected over 50 years (including some nice brand new jets). The jet marked 57 or 58 (if it is correct and not nicked nor damaged) is a good place to start with a single Rochester B. But I'll be honest...Having 2 Rochester B carburetors on a 235 is like having 2 broken legs. You aren't going to go very far nor very fast and the trip won't be pleasant at all. And as Jerry pointed out already the springs for your power pistons will be completely wrong for the available vacuum so you'll be running rich unless you're going downhill with your foot off the gas pedal. If you're still planning on going with a stock manifold and one carburetor, I would suggest getting a nice Carter YF or call Jon (carbking) and see what model of Zenith or possibly Carter W-1 (Jon likes those quite a bit) or other carburetor he would suggest. Parts for some will be more readily available than others. You'll spend some money on a Carter, more on a Zenith and maybe even more on some other alternatives (like Stromberg). If you're serious about using the Rochester B, I'd suggest at the very least straightening it. Doing that will solve a few problems, and as many times as most Rochester Bs have gone through the "rebuilder mixmaster," you may have some other problems (which I discovered in the ones I bought for the study). Here's some info on that: https://tinyurl.com/8pydzfd6. Good luck no matter what you choose.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Jon G - just one of many reasons I recommend the Carter W-1 for use in multiple-carburetor environments is that the power circuit is mechanical!!!
The position of the metering rod (with metering jet, a.k.a. power) is controlled by the position of the throttle shaft, not by vacuum. Thus one eliminates the headaches of winding special vacuum springs.
Jon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
Thanks to all for the amount of info on the subject Seems like I’m leaning back to stock manifold and the carburetor that’s been on and working without issues. Put the duel carb intake back in the garage for expensive garage art or for sale not sure on that one yet if I keep it I will save some money for a good set of carburetors as mentioned here and elsewhere. Again thanks to all. Robert
Dual carbs are not for everyone and they do require a bit of tinkering. They are certainly not a bolt on a go part so for the moment, it seems you have probably made the correct decision for yourself.
I got a chance it open up my carburetors Carburetor # 1 size 53 jet # 2 size 59 jet So what would be the best size jets to look for ? Another website a guy with same carburetors and intake manifold said his carburetor shop installed size 48 jets
Did you read the Pat Smith article that I linked for you? Pat gives a recommendation on where to start with main jets and other invaluable information on setting up dual Rochesters.
Did you read the Pat Smith article that I linked for you? Pat gives a recommendation on where to start with main jets and other invaluable information on setting up dual Rochesters.
Yes I did and went back and read it again I have access to a set of 49s and a set of 51s will start there
) that are oversize for the engine.